Wax-based hot melt adhesives or coatings have been used for many years. Hot melt adhesives are bonding and coating agents which achieve a solid state and resultant strength by cooling as contrasted with other adhesives which achieve the solid state through evaporation or removal of solvents. Prior to heating, a hot melt adhesive is a thermoplastic, 100% solid material. When heated to approximately 275.degree. to 400.degree. F., the material is brought to a liquid state, and after the heat is removed, it sets usually by simple cooling. Hot melts offer the possibility of instantaneous bonding, especially when joining previously uncemented and often smooth impermeable surfaces.
When a hot melt adhesive comes in close contact with a surface to be bonded, a molecular layer of film at the surface of this substrate immediately attains a temperature approaching that of the hot melt through conductive heat transfer. Furthermore, a high degree of wetting, almost coalescence, of the hot melt coating and the surface material occurs. Directly afterward, the melt loses heat to substrates disposed on opposite sides of the melt or to a single substrate located on one side of the coating and to the processing evnironment on the other side of the coating until a temperature equilibrium is attained. Normally in a hot melt adhesive coating process, the temperature of the entire adhesive substrate are the melt a solid having cohesive strength for a particular finished product end use.
In short, hot melt adhesives achieve a solid state and resultant strength through cooling, as contrasted with other adhesives which achieve the solid state through evaporation or removal of solvents or through polymerization and/or crosslinking. As has been noted, hot melts contain no solvents and are 100% thermoplastic.
The ideal hot melt adhesive would be solid at room temperature and capable of being stored and handled easily without blocking. Upon heating, it would melt sharply and flow freely. It would be stable even with prolonged heating and able to withstand local overheating. In liquid-melt form, it could be applied to the work by nozzle, wheel, wet coating head, or spray. When used for lamination, its point of solidification would be such that ample time would be availble to close the bond with only minimum pressure. Of course, bonds accomplished with this ideal hot melt would be strong and the range of materials which it could bond would be wide. Finally, the composition of the hot melt would be such that paper products bonded with the adhesive could be reclaimed by some simple process. These are general requirements. A quality that might be particularly desirable for one application could be unimportant for another. In any case, a hot melt adhesive should be in a liquid state at some elevated temperature in order to meet requirements as a hot melt; yet it must set to a cohesive solid state in the anticipated ambient temperature environment of the finished product containing the hot melt adhesive as a component.
Hot melt adhesive formulations vary widely depending upon raw materials, intended end use, properties and price considerations. Hot melt systems most often consist basically of a polymer and a diluent. The polymer is the essential ingredient and is the backbone or strength component of the hot melt adhesive. Almost any thermoplastic polymer with adequate resistance to heat degradation can be used. Examples of practical polymers are polyethylene, polyvinylacetate, ethylene-vinyl acetate (EVA) and ethylene-ethyl acrylate (EEA) copolymers. On the other hand, polyvinyl chloride and nitrocellulose are less practical because of their limited heat stability.
The molten viscosity of a polymer in a given system is proportional to its molecular weight. A hot melt adhesive could be 100% polymer, but there often would be two major drawbacks: limited adhesion and lack of molten properties such as tack, application temperature range and wetting ability.
The polymer as a component of hot melt adhesives, often produces low viscosity, low strength and poor mechanical properties when its molecular weight is relatively low. In contrast, a relatively high molecular weight polymer produces high viscosity, high strength and good mechanical properties. Because the properties of the polymer are a dominant factor in the performance of most hot melt adhesives, it is generally desirable to use relatively high molecular weight polymers in formulations.
The second component of hot melt adhesives is the diluent system. The diluent system makes it possible to utilize the properties of the polymer. It is usually a blend of materials such as wax, plasticizer, tackifying or plasticizing resin like wood rosin or rosin esters, stabilizers and sometimes inert pigments or extenders.
The diluent system provides a variety of effects. It is the vehicle for the polymer thereby lowering the viscosity of the hot melt and making it more convenient to apply. It enhances the wetting ability and adhesive strength of the polymer. It provides molten tack and tack range. It contributes barrier properties and gloss and provides a means of making the polymer rigid or flexible.
The diluent for a hot melt remains in the system when a hot melt is applied and is not dissipated or absorbed. Any hot melt bond reflects the composite properties of all its components, including the diluent. If in the interests of machinability and low cost, lower molecular weight polymers are used combined with a high percentage of diluents, the effectiveness of a polymer in imparting its characteristics to a hot melt adhesive is reduced. The desirable polymer properties of toughness, heat resistance and low temperature flexibility are diminished almost in direct proportion to the molecular weight and concentration of the polymer.
Prior art wax-based hot melt adhesives particularly useful for adhering facings to building products and thermal insulations are usually characterized by their high cost. Typically the hot melt adhesives that are based on low cost ingredients either do not provide a moisture vapor barrier and/or do not provide sufficient bonding strength. At times kraft facings for building products and thermal insulations have been coated with an asphalt based compound to provide bonding and vapor barrier requirements. However, such a facing is characterized by its tendency to block (e.g., for a layer of coated facing to stick to the layer below it) and potental environmental problems associated with the application of the asphalt coating or the use of the coated facing. The asphalt compounds of the past are thus not appropriate for certain applications.
Certain large carpet manufacturers utilize EVA based hot melt adhesives which include a high percentage of fillers, i.e., up to 60% by weight in order to reduce costs. However, these EVA based adhesives do not provide a moisture vapor barrier which is especially necessary in coating kraft paper for use in bonding facings onto insulations at the manufacturing location.
An example of barrier properties, especially for the frozen food field, is found in ethylene-vinyl acetate blends with petroleum waxes. These coatings which usually have good gloss and heat sealability are specifically designed as barrier coatings. However, here the primary desired features are coating properties and esthetic properties with less emphasis on an ability to bond a variety of substrates, molten tack, tack range or strength characteristics.
Prior art hot melt and asphalt based adhesives have a definite "tack" or stickiness at room temperature. When these have been used to coat sheet materials which are commonly rolled for storage or shipment (and are therefore hereinafter referred to as "roll goods") it has been necessary to take steps to prevent the sticky adhesive on one layer of the roll from adhering to the sheet which forms the next layer of the roll. Such means have included incorporating paper separators as part of the roll and/or dusting the sheet surface with materials such as talc, or coating the obverse side of the facing with a release agent. This, of course, adds substantial additional fabrication and material costs as well as incorporating into the roll goods materials such as separating paper and talc which are not needed for the end use of the roll goods and therefore must be removed and discarded by the user.
In many instances, the hot melt adhesives which are applied to sheets to form adhesive jacketing materials which are used as vapor barrier facings for building materials and thermal insulations, are installed outdoors or in other environments where they are subjected to humidity and/or low temperatures with resultant cold cracking. This cold cracking often caused hot melt adhesives to be rejected for use in many applications where they would otherwise have been quite suitable.